Device isolation technology is being employed for electrically isolating devices from each other on a semiconductor substrate in forming an integrated circuit. Representative examples of such device isolation technology include a LOCOS method and the so-called "BOX (Buried Oxide Isolation) method" wherein a recess is formed in the surface of a semiconductor substrate, then refilled with an insulator. The process of the LOCOS method is illustrated in FIGS. 8(a) to 8(d).
First, as shown in FIG. 8(a), a silicon nitride film 52, for example, is formed on a semiconductor substrate 51 (for example, made of n type silicon) by CVD method.
Next, as shown in FIG. 9(b), the silicon nitride film 52 except for that on an intended active region 53, is removed by etching using a resist film 54 as a mask, followed by ion implantation of, for example, boron without removing the resist film 54. Note that an arrow indicates a track of an ion's in the drawings. Since the resist film 54 prevents boron ion from reaching the silicon substrate 51, only a desired device isolation region 55 of the substrate 51 is implanted with boron ion. This ion-implanted region is a p.sup.+ -region which will serve as a channel stop region 56 for preventing a channel from forming in the device isolation region 55 due to occurrence of an inversion of the conductivity type at the surface of the silicon substrate 51 in that region.
In turn, as shown in FIG. 8(c), after removal of the resist film 54, a field oxide film 57 of SiO.sub.2 is formed on the region uncovered with the silicon nitride film 52 by thermal oxidation method.
Further, as shown in FIG. 8(d), the silicon nitride film 52 is removed by hot phosphoric acid to define on the silicon substrate 51 a device isolation region 55 with the field oxide film 57 and an active region 53 without the field oxide film 57. In addition, the channel stop region 56 is formed just under the field oxide film 57.
With the above LOCOS method, a protrusion 58 called a bird's beak is formed at opposite ends of the field oxide film 57. Forming the protrusion 58 in the active region 53 causes to narrow the active region 53. This results in such a problem that integration of devices cannot be highly increased.
Further, upon formation of the field oxide film 57 by thermal oxidation method, the impurity forming in the channel stop region 56 is likely to be absorbed by the field oxide film 57 due to segregation. Hence, the impurity concentration of the channel stop region 56 might be undesirably lowered to deteriorate a function of the channel stopper. This causes such a problem as to increase leakage current at channel stopper.
In addition, also upon formation of the field oxide film 57, the opposite ends of the silicon nitride film 52 are raised by the field oxide film 57 (refer to FIG. 8(c)), with the result that a strain is generated due to a stress in the silicon substrate 51 deposited with the silicon nitride film 52. Thus, crystal defect due to such strain tends to be formed.